Many applications require the measurement of temperature with a high degree of resolution. For example, in industrial/chemical process control a resolution of less than 0.1° C. may be desirable. In biological sensing applications, a resolution of less than 0.01° C. may be desirable. Temperature measurement may be performed on mammals, e.g., humans or animals, for example to determine mammalian ovulation timing by measuring a change in body basal temperature. This change is typically from about 0.05° C. to about 0.25° C. over 2 to 5 hours. Readings have to be taken over long periods of time so a low power temperature measurement system is required (e.g., 5-6 months lifetime with 16 readings/second). Additional applications include livestock monitoring and crop analysis.
High resolution measurement can be performed in a number of ways. For example, direct reading of resistance in the case of thermistors and resistive temperature devices (RTD) or direct reading of a derived measurement such as voltage using an analog-to-digital converter (ADC) may be used. When using either technique the resolution thereof may be enhanced with a high resolution Delta-Sigma analog-to-digital converters (ADC). To provide high resolution measurement of temperature, existing measurement solutions have used the Delta-Sigma ADC, however, use thereof typically limits the sampling rate to less than 500 Hz and causes problems in low power applications due to the conversion time and circuit power demand. Furthermore the delays and settling times inherent in such a temperature measurement system means that it must be turned on for long periods of time before a valid result can be generated, limiting thereby its use in a low-power system.
Furthermore, high resolution temperature measurement requires low noise, long sampling times and many temperature measurement observations. This limits the applicability to battery powered modules which must operate for many months. Existing techniques for measuring temperature using a counter and comparator provide a result limited directly by the resolution of the counter clock. All known existing methods reference this way of measuring temperature and suggest enhancing temperature measurement resolution by increasing the counter clock rate. However, increasing the counter clock rate has a resulting impact on the current consumed by the temperature measuring system and limits its applicability for use in low power systems, e.g., battery powered operation.